Now before you leave thinking that I am a lunatic, I certainly don't think it is good for the individual. I don't wish disease or illness on anyone and I certainly strive to avoid getting sick myself. That is not what I mean. I am talking more in the context of for humanity as a whole. This is still an odd premise, I will grant you, but if you bear with me, I believe it will make sense.
Let us start with another question: What prevents the populations of any species from continual exponential growth? I can think of only four things: predation, limited food, disease, and limited reproduction. In a world of limited resources, any species will multiply to a point where it's population growth is greatly slowed or halted by all four of them to a greater or lesser degree.
For most species on our planet predation is at the top of the list. Many species are limited by being either a food source or an inferior competitor to other species. This is what keeps rabbits and mice from completely taking over the planet. As a limit to population growth this is certainly effective, but it is also brutal and unpleasant. As humans we have few predators, since our intelligence, community and mastery of technology has allowed us to become superior (at least in terms of killing power and in the general sense) to all other species on the planet. If that were not the case, human existence would be much worse off. Imagine a world where there were superior predators who kept our population growth limited. Much of our lives would be devoted to survival and those who were slower, weaker, or otherwise less likely to survive would live in greater fear and horror. Or worse, and probably more likely, people would begin to control the system, by finding ways to sabotage either other into becoming the next victims. Humans are much better off to suffer very limited predation from another species.
Humans have through history feared predation from other humans as differing civilizations have clashed over resources or ideology. These wars have limited populations growth for humans on occasion, but thankfully warfare has not often been a primary limitation to the growth of human population. If there were not for the other limits on population growth, however, a world of constant warfare over limited resources would certainly be a very likely possibility.
Limited food is probably the largest growth limiter for early stage civilizations of humankind. While it extreme cases it lead to starvation, it often limited growth less harshly by reducing life expectancy and reproduction through both malnutrition and highly demanding lifestyles (often leading to very high infant and child mortality rates). As humans began to form civilizations, however, limited resources turned instead into warfare. Fortunately, civilization came largely because of increases in human ability to gather food allowing us to avoid (for the most part) the horrors of massive warfare for resources needed to survive. With that increase in available resources, however, human populations were able to expand rapidly. This would have lead quickly to just that type of warfare but for two things: expansion and disease.
Human expansion could delay the problem, but it was always limited by human ability to either get to or successfully live in new lands. Eventually, without another limiter humans would grow in population to fill all the usable land area. As technology increased, so does the usable land, but that would eventually have limits, and often times in our history, expansion could not occur fast enough to allow enough resources for the growing populations.
That brings us to disease. Don't get me wrong, disease is terrible. When you compare it to the alternatives, however, it doesn't look so bad. Disease limited massive populations from rising in small areas. If it weren't for disease, there would have instead been a mix of wide spread starvation and warfare(human predation) which would have limited populations in localized areas through history. It was disease which kept either of those things from happening.
Now you might argue that death is death, and that dying from disease is no better than being killed in war or starving. Looking at the individual, I would agree. Looking at the bigger picture, however, I do not. What makes disease preferable is that is semi-random, but that there are many things which can be done to reduce the odds, and these actions lead to less damaging moral decisions because it is largely out of human control. Starvation can be controlled by human decision and action to the point that it would lead to warfare followed by subjugation of the losing population (likely defined by ethnicity, culture, religion, or other generic trait). The subjugated population would suffer from starvation unless they were able to grow in power and overthrow the oppressing population at which time the roles would change and the cycle would likely repeat until something happened to either vastly decrease the population or vastly increase the available food. This would not only lead to terrible life for one people group or another, but leads greatly to the moral corruption as people would constantly be faced with deciding who gets to live and who has to die.
Disease, however, has been mostly out of control of humans for most of history. As I said earlier it affects individuals in a semi-random manner (at least to the perceptions of humans for most of history), there were things which could be done to limit risk, but not eliminate it. You couldn't control it to the point of directing it at any population group and you could not eliminate it to the point that it didn't reduce human population. It is the one population control that would not lead early civilizations to ruin.
I saved limited reproduction for last, because in the past hundred years or so, things have changed greatly. Technology has both greatly limited the impact of disease while at the same time allowing for far more limited reproduction. Limited reproduction is certainly the most desirable limit to population growth as it does not involve death. If it is present in a species too early, however, there is the large risk that the species will not expand, but rather become extinct. If humans took far longer to reproduce, it is unlikely that we would have survived. Certainly many civilizations would have been lost after major outbreaks of war, famine, or other large scale population reduction. Humans might never have had the localized numbers to cooperate and develop technologies which allowed them to overcome other species or expand their food supply to overcome starvation. Today, since we have already accomplished these things and have a large, widespread population, limited reproduction is the desirable course. In fact, we limited our population greatly, and some fear that in the cases of some nations, too much. Humanity, however, will survive especially since we are better mastering both birth control and limiting disease at the same time.
While the need for disease is largely passing, I believe it has been very beneficial to our past. I have not been able to come up with an alternative which would lead to a better outcome for us. Perhaps one of you will. I, however, believe that without disease the world would certainly be a different place, and I suspect that place would be much worse than the world we have today.
